This month we are going to finish looking at my room. In Sound Tips 4 & 5,
we addressed my speaker placement and acoustic treatments. We are now
going to look at the final issue, the icing on the cake, equalization.

Now, I know that word, “equalization,” strikes fear and loathing into
the hearts of many audiophiles, but hey, let’s get real. If you have a
home theater, you should know that almost every commercial movie
theater in existence uses EQ (equalization). Every dubbing stage (film
mixing studio) uses EQ. All of the places doing high-end film audio
remixes for DVD release, like Mi Casa Multimedia (which works with New
Line on films such as “The Lord of The Rings” trilogy), uses EQ. In
fact, the Mi Casa guys are so tweaky about the sound in their rooms
that I have a lifelong gig several times a year making sure the rooms
stay in tune.

So why, after all the trouble we went to, properly placing our speakers
and treating the room, would we need EQ? Basically, most rooms we
listen in are imperfect, even with acoustical treatment. There are rare
cases where I set up rooms that did not require EQ after the physical
work was finished, but those are one in a hundred. I do want be clear:
I do not believe in the indiscriminate use of EQ. I’m against adding
more electronics into the listening chain unless it improves
significantly more issues than it creates. However, most rooms that
were not designed for audio from the ground up have some problem or
other that is not treatable with just acoustic fixes. These problems
would include some situations that exist in my room: open walkways,
asymmetrical walls, asymmetrical furniture placement, etc. Then there
is the problem of dealing with bass in a fashion acceptable in most
living spaces. Treating bass problems requires more space than most
home owners are willing to sacrifice and there is the nagging issue of
the aesthetics of a bass trap. A well-built minimum phase parametric
equalizer can solve low-frequency problems cost effectively and with no
aesthetic intrusion in a room.

Many subwoofer manufacturers have recognized the above facts and are
now building EQ into their products. Even high-end companies like
Meridian are putting EQ and room correction as features in their AV
preamps. The age-old audiophile arguments against using EQ thankfully
seem to be finally fading away.

There are two styles of equalizers currently utilized for room tuning.
These are third octave (fixed frequency centers and bandwidth) and
parametric (adjustable frequency centers and bandwidth). I maintain
strong opinions on this subject, backed up by science and listening. I
recommend minimum phase parametrics over third octave equalizers.

To date, most equalizers utilized for tuning rooms have been third
octave models. At the time of their origin, people actually believed
that humans could not hear bandwidth narrower than third octave, so
those EQs were applied to room tuning in professional audio. We now
know that human hearing is much more sophisticated. The fixed nature of
the third octave equalizer is its primary limitation. In my opinion,
third octave EQ is now "old school.” With third octave EQ, “You can’t
get there from here.”

Room problems are not well-behaved phenomena that follow fixed
patterns. A parametric equalizer allows you to dial in the exact center
frequency to address the problem. Then it allows you to shape the curve
(bandwidth) to give a proper fit solution. Third octave is simply hit
or miss on the center frequencies and brute force with its fixed
bandwidth. We end up equalizing more than necessary or often can't get
to the problem at all with third octave equalizers. Parametric
solutions just make sense. Figures 1 & Figures 2
demonstrate the point. (Note that the green equalizer curve displayed
is the inverse of the equalization that is being applied. This simply
makes it easy to see how it fits into the room curve.) Figure 1
shows the low end of a room curve with a third octave solution applied
to it. Notice that you can't quite get to the problem and wind up
affecting more frequencies than necessary, especially around 150Hz. Figure 2 is the same room with a parametric solution. The parametric exhibits a much better match.

In the past, it was difficult to design parametrics that did not
introduce extraordinary phase shift, but today many designers have this
situation solved. Some room equalizers of modern design even utilize
minimum phase filters. Minimum phase frequency response anomalies occur
when speakers are placed in proximity to boundaries such as walls,
ceilings or soffets. Minimum phase filters allow for more latitude to
boost or cut without hearing that excess phase shift. It has also been
proven that they can restore the impulse response of a speaker affected
by minimum phase problems.

Equalizable room/speaker interaction is a minimum phase, second order
phenomenon, exhibiting constant bandwidth and linear frequency spacing.
At best, third octave equalizers are constant percentage bandwidth with
logarithmic frequency spacing. Figure 3
demonstrates the type of frequency response comb filtering caused by a
one-millisecond echo. In practical terms, this is typical of a
reflection from a coffee table sitting in front of the couch. Note that
the combing is not logarithmically spaced (e.g., third octave), but
rather a constant bandwidth of 1kHz. The frequency centers of the comb
are an octave wide from 1kHz-2kHz and 1/10 octave wide from
10kHz-20kHz, meaning that a device with fixed third octave bandwidth
cannot create a complement. They also fall on frequencies that often do
not match the fixed ISO standard frequency selections of a third octave
equalizer. Once again, parametrics give you the versatility to address
the problem as it exists. Parametrics also allow you to utilize a
single filter to address an overall trend in the response, while with
third octave, several filters must interact to address this same issue.
I am not advocating equalizing the coffee table reflection; this is
simply a practical demonstration of the problem.

Part of the blame for using third octave equalizers for so long must be
placed on the fact that the industry primarily used third octave
analyzers. This made it a simple match-up for tuning and is quite
understandable. But third octave analysis does not offer enough
resolution to actually see the whole story. There are now a number of
affordable analyzers on the market that have much higher resolution. My
SIM System crunches the numbers at 1/48th-octave resolution. Look at Figure 4 and Figure 5. Figure 4
is a third octave mid-band shot of a room. Notice that there appears to
be a hole from about 500-800Hz (highlighted area). This would lead one
to believe that some boost centered at 630Hz on your third octave
equalizer would fix the problem. In actuality, viewing the 1/24th
octave resolution of Figure 5,
we see that this is not a wide band hole but a series of tight combs
that should probably not be equalized. The third octave analyzer
averaged this section out in a broad stroke. This simplified but
accurate picture is the rule rather than the exception. I strongly
suggest that you look at your rooms with effective resolution if you
want to correctly solve the problems.

While many
people understand the need to "tune up," there is quite a bit of
confusion about maintaining the system tuning. There are several
contributory factors as to why a system needs to be checked
periodically to stay in tune. Some acoustic factors are relatively easy
to understand. If wall treatments or furniture have been added, then
you have changed the linearity of the system response, as well as the
room's reverb structure. The same would apply to construction, such as
adding an equipment rack or anything that changes wall angles, room
volume or mass.

Changing electronic components can also make a big difference in
linearity. New amplifiers may handle your speaker impedance or wire
capacitance differently and cause changes in linearity. I have
witnessed changes of several dB in different parts of the frequency
spectrum when clients switched between solid state and tube amps. Of
course, changing crossovers applies as well. Filter order and the
crossover point itself will have an effect on the system. Of course,
with any of these changes, attention must be paid to overall system
polarity.

Any time you change a speaker component, you should check the system
response. I have seen fluctuation by as much as 3dB across the range of
a speaker when an old element was replaced with a new one. Sensitivity
will vary and many manufacturers do not have a tight grip on quality
control in this department.

The above examples are fairly clearcut, as they are actual physical
changes that are easy to identify. Now we enter into a gray area. The
most asked question I hear is, "How often should I adjust the EQ in my
system?" It is a physical fact of life that, as speakers age, their
resonant frequencies change from fatigue. These resonances need to be
adjusted over time, as the tuning will "drift" out. How long that takes
depends on how often and loud a system is driven. Moderately-driven
systems can often go years before changes in linearity occur. Some
clients, like busy recording studios that operate their speakers 24
hours per day and listen often at loud volumes, may need attention
after six months.

So getting away from all the tech talk, how did my room finally turn out? Figure 6
shows my little EQ rack. I am using five channels of the Meyer CP-10
& 10S EQs for the Meyer HD-1 speakers on L/C/R/LS/RS. I use the EQ
built into the Revel B-15s for the LFE channel. Figure 7 shows the correction I applied to my right speaker. This is fairly typical of the correction performed in my room.

After equalization, I heard several significant improvements. The bass
was much tighter and better defined. The frequency spectrum was more
linear, which equates to no musical notes missing or outstanding; the
musical spectrum was smoothly connected from top to bottom. Imaging
improved as well, because now the speaker frequency responses were
extremely well-matched. Not only was it easier to pick out individual
instruments in the soundstage, but the center image was rock solid as
well. No more voice that sounds three feet wide.

Between the use of parametric equalizers and high-resolution analysis
systems, there exist good solutions to address room/speaker interface
correction. These solutions can significantly upgrade the quality of
the listening environment, leading to a more satisfying and involving
listening experience.

Bob
Hodas tunes many of the worlds finest recording studios and mastering
labs as well as many of the best private home theater and music
playback systems. His clients include: George Lucas, Abbey Road London,
Sony Music Tokyo, Paul Stubblebine Mastering and many more http://www.bobhodas.com/clients.html.

Based in Berkeley California, Hodas travels the world to tune audio
systems, designs acoustically excellent rooms and implements acoustical
treatments. Bob is available for consultation as well as in-home
tunings starting at $500. To contact Bob Hodas, email bobhodas@bobhdas.com or call (510) 649-925